The present invention generally relates to an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation which may result from the mistimed delivery of cardioverting electrical energy to the atria of the heart. More specifically, the atrial defibrillator of the present invention guards against applying cardioverting electrical energy to the atria of the heart under conditions believed to contribute to induced ventricular fibrillation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has precluded these defibrillators from becoming a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in copending U.S. application Ser. Nos. 07/685,130, filed Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness for IMPROVED ATRIAL DEFIBRILLATOR AND METHOD and Ser. No. 07/856,514, filed Mar. 24, 1992, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for IMPROVED ATRIAL DEFIBRILLATOR, LEAD SYSTEMS, AND METHOD, which applications are assigned to the assignee of the present invention and incorporated herein by reference. As disclosed in the aforementioned referenced applications, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart is important to prevent induced ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The atrial defibrillators of the aforementioned referenced applications exhibit improved safety from inducing ventricular fibrillation by sensing ventricular activations of the heart in a manner which avoids detecting noise as ventricular electrical activations for generating reliable synchronization signals. Hence, these implantable atrial defibrillators, by providing such noise immunity in R wave detection assure reliable synchronization.
Another measure for reducing the risk of inducing ventricular fibrillation during the delivery of cardioverting electrical energy to the atria of the heart employed by the defibrillators of the aforementioned referenced applications is the reduction of the amount of the electrical energy which is passed through the ventricles during cardioversion of the atria. This is achieved by locating the cardioverting electrodes in or near the heart to provide a cardioverting energy path which confines most of the cardioverting electrical energy to the atria of the heart.
The atrial defibrillator and method of the present invention provides a further improvement to the end of safety and reduction in the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation. It has been observed that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced from the T wave of the immediately preceding cardiac cycle. This may lead to a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with the R wave close to the T wave. During highly variable cardiac rates, a long cardiac cycle can be followed by a relatively short cardiac cycle. This condition in conjunction with a high cardiac rate is believed to cause a dispersion of refractoriness and also can result in an increased vulnerability to ventricular fibrillation. For a more complete understanding of the aforementioned highly variable cardiac rate and the consequences thereof, reference may be had to an article entitled El-Sherif et al., Reentrant Ventricular Arrhythmias in the Late Myocardial Infarction Period: Mechanism by Which a Short-Long-Short Cardiac Sequence Facilitates the Induction of Reentry, Circulation, 83(1): 268-278 (1991).
The atrial defibrillator and method of the present invention greatly reduces the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation by avoiding applying the cardioverting electrical energy to the atria at those instances when increased vulnerability to ventricular fibrillation may be present. As will be seen hereinafter, this is accomplished by interval timing prior to applying the cardioverting or defibrillating electrical energy. The time interval between immediately successive R waves is timed and the cardioverting or defibrillating electrical energy is only applied when a timed interval is greater than a preselected minimum interval. This provides protection for the increased vulnerability to ventricular fibrillation condition resulting from a high cardiac rate. To provide protection for the R on T condition resulting from a highly variable cardiac rate, a further condition may be applied to the timed interval requiring the timed interval to also be less than a preselected maximum interval before the cardioverting or defibrillating energy is applied to the atria.